This invention is directed to apparatus for the application of a coating to an object by thermal spraying and in particular to an apparatus which heats and propels powder particles by a high current, transient discharge plasma arc.
Of the several thermal processes, the detonation gun produces the highest quality coating. This type of apparatus is described in the publication by T. C. McGeary and J. M. Koffshey entitled "Engineering Applications for Flame Plating" -- Metal Progress, Jan. 1965. This process is essentially a modification of the steady combustion spraying process in that chemical energy is released periodically in the form of detonation waves. The pressure, temperature and velocity of the detonation product are substantially higher than in the steady process, resulting in higher velocity powder particles and thus a better quality and more adherent coating. However, it has been found that this process is very noisy and can result in oxidized coating.
A process which avoids this problem, is the steady plasma arc process, examples of which are described in U.S. Pat. No. 3,246,114 issued Apr. 12, 1966 to Matvay and U.S. Pat. No. 3,346,698 issued Oct. 17, 1967 to Ingham Jr. These devices use a steady plasma flame as the heat source in the spraying process to heat and propel the powder particles. However, it has been found that the temperature and pressures cannot be raised to high levels since the plasma confining walls tend towards a thermal equilibrium with the plasma resulting in heat loss and the electrodes have a tendency to erode.
These disadvantages may be overcome by apparatus in which a pulsed mode of arc heating is utilized since higher temperatures and velocities may be achieved. One such apparatus is described in U.S. Pat. No. 3,212,914 issued to Lyle et al. on Oct. 19, 1965 which provides for a high temperature, high velocity coating process that employs a pulsed electric discharge in an inert gas atmosphere as the coating energy source. However, to date, apparatus that has found acceptance over the detonation gun described above, has not been developed.